Penetration grade bitumen

The actual performance of the bitumen depends on the total performance of a bitumen / aggregate mixture which again depend as the design of the mixture, the way it was mixed and how it was laid and compacted.

Minimise the time during which hot bitumen is exposed to air. Bitumen hardens and loses some of its adhesive property when it is exposed to a combination of air and heat.

Careless mixing of bitumen and aggregate is an example of a practice that exposes bitumen to the air and heat for longer than the minimum time required to coat the aggregate. it is therefore likely to reduce the bitumen's performance as an adhesive.

Keep bitumen dry and away from contamination by water at all times. When it comes into contact with water it will foam.

Keep bitumen within the range of temperatures recommended for the type and grade.

Bitumen solidifies as it cools and becomes unworkable, and the viscosity may increase if it is overheated. It is sensible to use bitumen at the lowest temperature that will permit it to be mixed, transported and compacted but in any case the maximum storage temperature relating to the grade should not be exceeded during the mixing process.

Temperature control is a major factor in the quality control of a bitumen / aggregate mixture. Avoid contaminating the bitumen, particularly with petroleum based solvents that reduce the Flashpoint and the viscosity and increase the penetration. just 0.1% diesel oil in a bitumen / aggregate mixture can lower the flash point and increase the penetration significantly.

Stability is an essential quality in the performance of a bitumen / aggregate mixture. it depends on friction and cohesion within the mixture. Too much bitumen actually acts as a lubricant for the aggregate particles, reducing friction and therefore the mixture's stability. The cohesion or internal binding force in the mixture increases as the proportion of bitumen in the mixture is increased, until it reaches the optimum. if the amount of bitumen is increased further, stability decreases. it is therefore essential to ensure that the optimum proportion of bitumen is used in any mixture.

The following are useful examples of what can be seen by simple observation. It can be treated as early warning signs that further checks should be made:

Smoke coming from the mix indicates overheating

Steam from the mixture indicates the presence of moisture

If the distribution of bitumen on the aggregate is not uniform, the temperature in the mixing operation was too low

if the mixture looks stiff and dull rather than black and shiny there may be too little bitumen in the mixture

if the mix lies slumped on the floor of the truck there may be too much bitumen in it